1. Technical Field of the Invention
The present invention relates generally to the control of vibration transmission and more particularly to the control of vibrational energy propagating or transmitting within structural elements such as beams and more complicated frameworks comprised of such elements.
2. Discussion of the Related Art
In many aerospace and marine structures, vibration generated by attached machinery is a significant problem. Often these structure-borne vibrations travel through the structure in the various forms of flexural, extensional and torsional waves and ultimately radiate as undesirable sound. Passive techniques such as constrained layer damping, involving bonding a visco-elastic material on the structure to dissipate vibrational energy, and the more traditional addition of mass and/or stiffness to the structure have been used in an attempt to reduce this sound. However, these techniques have an inherent weight penalty and are often ineffective in the low frequency region. It is in this situation that active control approaches have shown promise. For example, it is known that active forces applied to beam-like structures can attenuate bending waves and the associated flexural power in the media. Also, flexural power flow along finite beams terminated by an arbitrary impedance, which is representative of any structural termination, can be blocked using a single control input. This is in contrast to the usual model approach of attempting to reduce the total vibrational energy in the beam section. An efficient control approach can be implemented if vibrational energy from, e.g., machinery can be confined to the immediate supporting structure prior to dispersion through the overall system.
As mentioned previously, vibrational energy is carried by flexural, extensional and torsional wave forms. It has been shown that significant wave conversion can occur at structural joints due to the coupling inherent in the boundary conditions. For example, extensional waves, although having small out-of-plane displacments, may carry a large amount of energy due to the structure being much stiffer for in-plane motion. On impinging on a discontinuity, these extensional waves may couple strongly to flexural waves with large out-of-plane motion and subsequently radiate large sound levels. Thus it may be deduced that all forms of vibrational waves are important in terms of control of structure-borne sound. Nonetheless, all previous work on control of vibrations in beams appears to have dealt solely with flexural motion.